The use of tin based solder alloys is common in electronic applications, particularly in the manufacturing of printed circuit boards (PCB), for assembly of components onto the boards, providing mechanical and electrical connection. These tin solder alloys are useful in joining integrated circuit chips to chip carriers and substrates, joining chip carriers to substrates, and joining circuitization lands and pads in multilayer printed circuit boards.
In the manufacturing of a microelectronic package, it is common practice to attach a component onto a printed circuit board or the like, for example by surface mounting utilizing a solder connection. For this purpose, the board features a circuit trace including a pad that constitutes a first surface for the connection; similarly, the component includes a second surface, for example a contact.
The interconnection method comprises the steps of applying a solder alloy on the Cu substrate, typically onto the pad included in the printed circuit board.
The electronic components to be joined with the board are then brought into contact with the solder layer. The solder alloy is heated to cause the solder alloy to melt and reflow; heating may be by vapor phase reflow, infrared reflow, laser reflow, or the like. Upon cooling, the solder alloy resolidifies and bonds to the surfaces to complete the connection. The solder connection not only physically attaches the component to the board, but also electrically connects the trace on the board and the contact of the component to conduct electrical current to and from the component for processing.
Tin-lead (Sn--Pb) alloys have been used for most electronic soldering operations. These alloys have been selected because of their mechanical strength, low relative cost, electrical conductivity and excellent wetting characteristics; wettability is an indication of how completely and quickly the molten solder can cover a solid surface. In addition, Sn--Pb alloys provide a low melting temperature, which is important in electronic applications because many components and printed circuit boards use materials that are easily damaged by exposure to high temperature during manufacture or assembly.
However, lead has been recognized as a health hazard, being toxic for workers and for the environment; recently governments have begun to urge the electronic industry to find alternatives to lead in order to reduce electronic industry worker lead exposure and reduce the amount of lead waste going back into the environment.
Lead presence in the soldering alloys is particularly critical in the case of application for manufacturing the most recent generation of C-MOS; in fact the details are so thin in this kind of board, that the emission of .alpha. particles from the emitting radioisotope present in the lead can provoke serious problems for the device.
Tin-Bismuth (Sn--Bi) solder alloys were investigated as alternatives to Sn--Pb solder alloys. Electrodeposition of such Sn--Bi alloys from different electrolytes and in particular from alkyl sulphonate baths is known in the art, as described in Surf. & Coat. Tech--Vol. 27, 151-166 (1986)--Y. N. Sadana, R. N. Gedye, S. Ali. Electrodeposition of Sn--Bi alloys onto a PCB with an alkyl sulphonate electrolyte is also described in U.S. Pat. No. 5,039,576.
A different lead-free solder alloy for microelectronic applications is described in EP-A 94108684.5. Such document discloses solder alloys containing more than 90% weight percent tin (Sn), and an effective amount of silver (Ag) and bismuth (Bi), optionally with Antimony (Sb) or with Sb and copper (Cu). Different methods for obtaining the described alloys, including electrodeposition, are mentioned.
Lead-free solder alloys known in the art present however some drawbacks. They exhibit poor soldering and metallurgical properties, that is small peel strength and low creep resistance. Particularly, they have shown poor mechanical properties at temperatures of the type typically encountered by microelectronic packages during use. A Sn--Bi alloy, for example, when electrodeposited onto copper of PCBs from alkylsulphonate or other electrolytes shows some difficulties related to the low wettability and stability.